The Rock Cycle
 Igneous rocks form from molten rock (magma)
 Characteristics of magma
 Protolith of igneous rocks
 Forms from partial melting of rocks
 Magma at the surface is called lava
 Characteristics of magma
 Rocks formed from lava are classified as extrusive, or volcanic rocks
 Rocks formed from magma are termed intrusive, or plutonic rocks
 The nature of magma
 Consists of three components:
 A liquid portion, called melt, made of mobile ions
 Solids, if any, are crystallized silicate minerals
 Volatiles, (dissolved gases), mostly water (H
2
O), carbon dioxide (CO
2
), and sulfur dioxide (SO
2
)
 Crystallization of magma
 Texture = the size and arrangement of mineral grains
 Igneous rocks are classified by
 Texture
 Mineral composition
 Texture describes the size, shape, and arrangement of interlocking minerals
 Factors affecting crystal size
 Rate of cooling
 Slow rate promotes fewer and larger crystals
 Fast rate forms many small crystals
 Very fast rate forms glass
 Types of igneous textures
 Aphanitic (fine-grained) texture
 Rapid rate of cooling
 Microscopic crystals

 May contain vesicles
 Phaneritic (coarse-grained) texture
 Slow cooling
 Crystals can be seen
Aphanitic texture
Phaneritic texture
 Types of igneous textures
 Porphyritic texture
 Minerals form at different temperatures as well as differing rates
 Large crystals (phenocrysts) embedded in a matrix (groundmass)
 Glassy texture
 Very rapid cooling
 Results in obsidian
Porphyritic texture
Andesite porphyry
Porphyritic Texture
Glassy (vitreous) texture
An obsidian flow in Oregon
 Types of igneous textures
 Pyroclastic texture
 Fragments ejected from violent volcanic eruption
 Textures similar to sedimentary rocks
 Pegmatitic texture
 Exceptionally coarse grained
 Late crystallization stages of granitic magmas
 Igneous rocks are primarily silicate minerals
 Dark (ferromagnesian or mafic) silicates
 Olivine
 Pyroxene
 Amphibole
 Biotite mica
 Igneous rocks are primarily silicate minerals
 Light (felsic) silicates

 Feldspars
 Muscovite mica
 Quartz
 Granitic versus basaltic compositions
 Granitic composition
 Composed of light-colored silicates
 Felsic composition
 High silica (SiO
2
)
 Major constituents of continental crust
 Granitic versus basaltic compositions
 Basaltic composition
 Composed of dark silicates and calcium-rich feldspar
 Mafic composition
 More dense than granitic rocks
 Comprise the ocean floor as well as many volcanic islands – the most common volcanic rock
Gabbro
Basalt
 Other compositional groups
 Intermediate (or andesitic) composition
 At least 25% dark silicate minerals
 Associated with explosive volcanic activity
 Ultramafic composition
 Rare composition, very high in magnesium and iron
 Composed entirely of ferromagnesian silicates

Mineralogy of common igneous rocks
Silica content influences a magma’s behavior
 Granitic magma (high silica)
 Extremely viscous
 Higher temperatures Molten as low as 700 o C
 Basaltic magma (low silica)
 Fluid-like behavior
 Pyroclastic rocks
 Eruptive fragments
 Varieties
 Tuff – ash-sized fragments
 Welded tuff – ejected hot, “welds” on landing
 Volcanic breccia – particles larger than ash (similar to sedimentary breccia)
Ash and pumice layers
Classification of igneous rocks

 Several Factors Involved
 Generating magma from solid rock
 Partial melting crust and upper mantle
 Role of heat
 Temperature increases with depth in the upper crust (called the geothermal gradient, ~ 20 o C to 30 o C per km)
Estimated temperatures in the crust and mantle
 Role of heat
 Rocks in lower crust and upper mantle near melting points
 Additional heat (from descending rocks or rising heat from the mantle) may induce melting
 Role of pressure
 Increase in confining pressure increases rock melting temperature and reducing the pressure lowers the melting temperature
 When confining pressures drop, de-compression melting occurs
Decompression melting
 Role of volatiles
 Volatiles (primarily water) lower rock melting temperatures
 Particularly important with descending oceanic lithosphere
 A single volcano may extrude lavas exhibiting very different
 compositions
Bowen’s reaction series and the composition of igneous rocks
 N.L. Bowen demonstrated that as a magma cools, minerals crystallize in order based on their melting points
 Bowen’s reaction series
 During crystallization, the composition of the liquid portion of the magma continually changes
 Composition changes due to removal of elements by earlier-forming minerals
 The silica component of the melt becomes enriched as crystallization proceeds
 Minerals in the melt can chemically react and change

Processes responsible for changing a magma’s composition
 Magmatic differentiation
 Separation of crystals from melt forms a different magma composition

 Assimilation
 Incorporation of foreign matter (surrounding rock bodies)
 Magma mixing
 Involves two magmas intruding one another
 Two distinct magmas may produce a composition quite different the originals
Assimilation and magmatic differentiation
 Partial melting and magma formation
 Incomplete melting of rocks is known as partial melting (silica rich minerals melt first)
 Formation of basaltic magmas
 Most originate from partial melting of ultramafic rock in the mantle
 Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones
Decompression melting
 Formation of basaltic magmas
 As basaltic magmas migrate upward, confining pressure decreases which reduces the melting temperature
 Large outpourings of basaltic magma are common at Earth’s surface
 Formation of andesitic magmas
 Interactions between mantle-derived basaltic magmas and more silicarich rocks in the crust generate magma of andesitic composition
(assimilation)
 Andesitic magma may also evolve by magmatic differentiation (loss of early olivine and pyroxene)
 Partial melting and magma formation
 Formation of granitic magmas
 Most likely the end product of an andesitic magma
 Granitic magmas are higher in silica and therefore more viscous
 Because of viscosity, mobility lost before reaching surface (rhyolite rarer than basalt)
 Tend to produce large plutonic structures
Magmatic Differentiation
 Many important sources of metals are produced by igneous processes
 Igneous mineral resources can form from
 Magmatic segregation – separation of heavy minerals in a magma chamber

 Hydrothermal solutions - Originate from hot, metal-rich fluids that are remnants of the late-stage magmatic process
Origin of hydrothermal deposits
 Factors determining the “violence” or explosiveness of a volcanic eruption
 Composition of the magma
 Temperature of the magma
 Dissolved gases in the magma
 The above three factors actually control the viscosity of a given magma which in turn controls the nature of an eruption
 Viscosity is a measure of a material’s resistance to flow (e.g., Higher viscosity materials flow with great difficulty)
 Factors affecting viscosity
 Temperature - Hotter magmas are less viscous
 Composition - Silica (SiO
2
) content
Higher silica content = higher viscosity
(e.g., felsic lava such as rhyolite)
 Factors affecting viscosity continued
 Lower silica content = lower viscosity or more fluid-like behavior (e.g., mafic lava such as basalt)
 Dissolved Gases
 Gas content affects magma mobility
 Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure
 The violence of an eruption is related to how easily gases escape from magma
 Factors affecting viscosity continued
In Summary
 Fluid basaltic lavas generally produce quiet eruptions
 Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions
 Lava Flows
 Basaltic lavas are much more fluid

 Types of basaltic flows
 Pahoehoe lava (resembles a twisted or ropey texture)
 Aa lava (rough, jagged blocky texture)
 Dissolved Gases
 One to six percent of a magma by weight
 Mainly water vapor and carbon dioxide
A Pahoehoe lava flow
A typical aa flow
 Pyroclastic materials – “Fire fragments”
Types of pyroclastic debris
 Ash and dust - fine, glassy fragments (<2 mm)
 Pumice - porous rock from “frothy” lava
 Lapilli - walnut-sized material (2-64 mm)
 Cinders - pea-sized material
 Particles larger than lapilli (>64 mm)
 Blocks - hardened or cooled lava
 Bombs - ejected as hot lava
A volcanic bomb
 General Features
 Opening at the summit of a volcano
 Crater - steep-walled depression at the summit, generally less than 1 km in diameter
 Caldera - a summit depression typically greater than 1 km in diameter, produced by collapse following a massive eruption
 Vent – opening connected to the magma chamber via a pipe
 Types of Volcanoes
 Shield volcano
 Broad, slightly domed-shaped
 Composed primarily of basaltic lava
 Generally cover large areas
 Produced by mild eruptions of large volumes of lava
 Mauna Loa on Hawaii is a good example
Shield Volcano
 Types of Volcanoes continued
 Cinder cone

 Built from ejected lava (mainly cinder-sized) fragments
 Steep slope angle
 Rather small size
 Frequently occur in groups
Sunset Crater – a cinder cone near Flagstaff, Arizona
Paricutin
 Types of volcanoes continued
 Composite cone (Stratovolcano)
 Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St.
Helens)
 Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base)
 Composed of interbedded lava flows and layers of pyroclastic debris
A composite volcano
Mt. St. Helens – a typical composite volcano
Mt. St. Helens following the 1980 eruption
 Composite cones continued
 Most violent type of activity (e.g., Mt. Vesuvius)
 Often produce a nueé ardente
 Fiery pyroclastic flow made of hot gases infused with ash and other debris
 Move down the slopes of a volcano at speeds up to 200 km per hour
 May produce a lahar, which is a volcanic mudflow
A size comparison of the three types of volcanoes
St. Pierre after Pelee’s Eruption
 Pyroclastic flows
 Associated with felsic & intermediate magma
 Consists of ash, pumice, and other fragmental debris
 Material is ejected a high velocity
 e.g., Yellowstone Plateau
 Steep walled, roughly circular, depressions at the summit of a volcano
 Size generally exceeds 1 km in diameter
 Formed by the collapse of the structure

Caldera Formation
 Type 1 Empty magma chamber under volcano leads to collapse
(like Crater Lake)
 Type 2 Magma chamber empties laterally through lava tubes
(like Mauna Loa)
 Type3 Magma chamber produces ring fractures and empties with explosive eruption producing silica-rich deposits and very large caldera (like Yellowstone, and Long Valley)
Crater Lake, Oregon is a good example of a caldera
Long Valley Caldera
 Eruptions from 3.8 to 0.8 Ma (from basalt to rhyolite) ended with Glass
Mountain eruption (0.76 Ma) that formed ring-structure caldera and produced the Bishop Tuff
 More recent Mono-Inyo Crater system (0.4 to present, also from basalt to rhyolite) includes Mammoth Mountain (made of a series of lava domes and flows)
 Currently active, probably reflecting smaller magma bodies
Long Valley Caldera
Bishop Tuff
Mono-Inyo Craters Activity
Explanation of Mono-Inyo Craters Trend
 Fissure eruptions and lava plateaus
 Fluid basaltic lava extruded from crustal fractures called fissures
 May travel as much as 90 km from source
 e.g., Columbia River Plateau individual flows accumulate to more than a km thick
Fissure Eruptions
 Lava Domes
 Bulbous mass of congealed lava formed post eruption
 Most are associated with explosive eruptions of gas-rich magma

 Volcanic pipes and necks
 Pipes are conduits that connect a magma chamber to the surface
A lava dome on Mt. St. Helens
Kimberlite Pipe
 Volcanic pipes and necks continued
 Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone
Formation of a volcanic neck
 Most magma is emplaced at depth in the Earth
 An underground igneous body, once cooled and solidified, is called a pluton
 Classification of plutons
 Shape
 Tabular (sheetlike)
 Massive (massive refers to shape, not size)
 Classification of plutons continued
 Orientation with respect to the host (surrounding) rock
 Discordant – cuts across sedimentary rock units
 Concordant – parallel to sedimentary rock units
 Types of intrusive igneous features
 Dike – a tabular, discordant pluton
 Sill – a tabular, concordant pluton (e.g., Palisades Sill in New
York), uplifts overlying rock, so must be a shallow feature
 Laccolith
 Similar to a sill
 Lens or mushroom-shaped mass
 Arches overlying strata upward

 Intrusive igneous features continued
 Batholith
 Largest intrusive body
 Discordant and massive
 Surface exposure of 100+ square kilometers (smaller bodies are termed stocks)
 Frequently form the cores of mountains like our Sierra Nevada
Batholith
Intrusive Igneous Structures
Batholith Distribution
 Global distribution of igneous activity is not random
 Most volcanoes are located within or near ocean basins (e.g. circum-Pacific “Ring of Fire”)
 Basaltic rocks are common in both oceanic and continental settings, whereas granitic rocks are rarely found in the oceans
Distribution of some of the world’s major volcanoes
 Igneous activity along plate margins
 Spreading centers
 The greatest volume of volcanic rock is produced along the oceanic ridge system
 Mechanism of spreading
 Lithosphere pulls apart
 Less pressure on underlying rocks
 Results in partial melting of mantle
 Large quantities of basaltic magma are produced
 Igneous activity along plate margins
 Subduction zones
 Occur in conjunction with deep oceanic trenches
 Descending plate partially melts (fluxed with water)
 Magma slowly moves upward
 Rising magma can form either
 An island arc if in the ocean
 A volcanic arc if on a continental margin
 Subduction zones
 Associated with the Pacific Ocean Basin

Region around the margin is known as the “Ring of Fire”

Most of the world’s explosive volcanoes are found here
 Intraplate volcanism
 Activity within a tectonic plate

 Intraplate volcanism continued
 Associated with plumes of heat in the mantle (perhaps from the core-mantle boundary, de-compression melting after rise)
 Form localized volcanic regions in the overriding plate called a hot spot
 Produces basaltic magma sources in oceanic crust (e.g., Hawaii)
Volcanic Activity
Volcanic Activity
Key Terms Chapter 6
Volcano (shield, stratovolcano, composite cone, cinder cone)
Lava
Magma
Pyroclastic
Pyroclastic flows
Viscosity
Fractional melt, fractionation, fractional crystallalization
Crystallization
Volcanic and plutonic rocks
Pluton (batholith, stock, laccolith, sill, dike)